The present invention relates to an optical fiber collimator in which a lens and an optical fiber are combined in order to collimate light emitted from an optical fiber into parallel light rays or to converge the parallel light rays into the optical fiber. The optical fiber collimator is useful in the case where light rays need to be kept parallel particularly in a long distance, for example, in an optical device having a structure in which parallel light rays pass through any kind of optical function device.
In the optical communication field, there is used an optical device in which two optical fiber collimators are disposed at a distance from each other and opposite to each other and an optical function device is disposed between the two optical fiber collimators so that parallel light rays pass through the optical function device. When light rays need to be kept parallel over a long distance, the beam diameter needs to be made large in accordance with the structure of the optical function device. Therefore, a long focal length lens is generally used in each of the optical fiber collimators.
On the other hand, in an optical device using propagation of light through space as described above, it is regarded as a matter of common knowledge that an end surface of an optical fiber is formed as an inclined surface in order to suppress feedback return light. In most cases, an obliquely polishing treatment is made.
The kind of the lens used in such an optical fiber collimator is not particularly limited but a gradient index rod lens which is columnar can be easily combined with an optical fiber chip holding an optical fiber. This is because it is easy to make an arrangement for making the center axis of the rod lens coincident with the optical axis of the optical fiber.
Light emitted from an optical fiber having an end surface treated to be inclined has an angle with respect to the optical axis because of the refractive index difference between the optical fiber and space. Hence, the center of a light beam incident on the lens slips out of the center of the lens. It is therefore necessary to use a lens having a large effective diameter. Hence, there is a disadvantage in that the outer diameter of the optical fiber collimator must be large. This is because insertion loss increases in a small-diameter lens owing to light beam shading and aberration loss generated.
An object of the invention is to provide an optical fiber collimator in which light rays can be kept parallel over a long distance and which can be reduced in diameter in spite of low insertion loss so that the effective diameter of a lens can be used efficiently.
According to the invention, there is provided, for example, an optical fiber collimator having a lens; and an optical fiber chip arranged at a distance from the lens, the optical fiber chip holding an end portion of an optical fiber and having an end surface treated to be inclined. Here, according to the invention, an optical axis of the optical fiber is configured to be made eccentric with respect to a center of the lens to thereby set a quantity of eccentricity of the optical fiber so that the center of the lens substantially coincides with a center of a light beam incident on the lens.
The kind of the lens is optional. The lens may be an inexpensive spherical lens or may be a gradient index rod lens. When a gradient index rod lens is used, a lens in which a surface facing the optical fiber chip is treated to be inclined may be preferably used as the gradient index rod lens so that the inclined surface of the lens is set to be substantially parallel (but not necessarily completely parallel) with the inclined end surface of the optical fiber chip. Further, the optical axis of the optical fiber is made eccentric with respect to the center of the rod lens so that the eccentric quantity of the optical fiber is set so that the center of the rod lens substantially coincides with the center of a light beam incident on the rod lens from the optical fiber.
As a typical example of the invention, there is a cylindrical member in which the lens and the optical fiber chip are incorporated in the cylindrical member so that the center of the lens coincides with the center of the optical fiber chip in a condition that the optical fiber is inserted and held in an optical fiber insertion hole formed in an eccentric position of the optical fiber chip. Alternatively, there is provided another cylindrical member which has a lens holding hole and an optical fiber chip holding hole formed so that the axes of the holding holes are shifted from each other, the lens and the optical fiber chip being inserted and fixed in the holding holes respectively to thereby be incorporated in a cylindrical member so that the optical fiber chip is made eccentric with respect to the center of the lens in a condition that the optical fiber is inserted and held in an optical fiber insertion hole formed in a center of the optical fiber chip.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2000-395902 (filed on Dec. 26, 2000), which is expressly incorporated herein by reference in its entirety.